The sealant flow and rim seal at the periphery of cavities prevent high-temperature gas from ingesting into disk cavities and reduce secondary air bleeding from a compressor. The unsteady sealing performance of rim seal and cooling characteristics on endwall of a 1.5-stage axial turbine are studied by solving the three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) equations via shear stress transfer (SST) turbulence model. Besides, mass transfer analogy is introduced to predict the sealant distribution. The numerical sealing effectiveness in front and aft cavities agrees well with experimental data and the accuracy of the numerical method is verified. The unsteady sealing effectiveness in cavities and flow field in rim seal clearance are analyzed, and the cooling performance on endwall is explored. The results show that, the sealing effectiveness increases with non-dimensional sealant rate and the sealing effectiveness of the rotor disk is higher than that of the stator disk under same sealant flow rate for front and aft cavities. Non-axisymmetric pressure on the endwall near rim seal and Kelvin Helmholtz unstable vortices arise due to the presence of blades and the circumferential velocity difference between annulus and cavity, and the flow field in the rim seal clearance is jointly affected by these two factors. According to FFT, the most significant frequency of flow field variety is about 1200Hz and 400Hz in front and aft rim seals, respectively. The cooling effectiveness increases with the sealant flow on endwall and decreases gradually in the axial direction.